Pressure sensitive adhesives are widely used for fabricating paper labels, laminating polymeric films such as laminating poly(vinyl chloride), polyester, etc., for forming decals, repositionable notes and other related products. Most of the pressure sensitive adhesives have been solvent based but in recent years government regulations have forced the industry to develop environmentally friendly aqueous based pressure sensitive adhesives.
Aqueous based pressure sensitive adhesives comprised of polymerized monomer units of vinyl acetate and ethylene (VAE), and/or acrylic esters, etc. have been inferior to the solvent based pressure sensitive adhesives in that they do not have sufficient dry tack or cohesive strength and the lack of desirable properties limits the use of these polymers as pressure sensitive adhesives.
There is a wide variety of applications for pressures sensitive adhesives and by pressure sensitive it is meant that that the polymer exhibits dry tack and will instantaneously adhere to a substrate with finger tip pressure at temperatures ranging from -20 to 25.degree. C. Most applications for permanent type pressure sensitive adhesives require excellent peel, tack and shear. Repositionable adhesives may require less tack but sufficient tack and cohesive strength to adhere to a substrate and yet be able to be removed without a portion of the adhesive adhering to the substrate. These pressure sensitive adhesives should also be resistant to oozing from the substrate when applied thereto and placed under pressure as in roll stock. Aqueous emulsion pressure sensitive adhesives must also be able to be coated on the various adhesive substrates such as Mylar, polyvinyl chloride and silicon containing coated polymers.
The following patents describe a variety of emulsion polymers incorporating vinyl esters and their use as adhesives:
U.S. Pat. No. 5,276,084 discloses aqueous based pressure sensitive adhesives based on polymers comprising vinyl acetate, ethylene, alkyl acrylates dialkyl maleate and fumarates, hydroxyalkyl acrylates, acrylamide and unsaturated carboxylic acids such as acrylic acid. The polymers are low in ethylene content, e.g., 5 to 30% by weight of the polymer.
U.S. Pat. No. 4,694,056 discloses vinyl acetate based pressures sensitive adhesives which comprise vinyl acetate, dioctyl maleate and acrylic acid polymerized in the presence of polyvinyl alcohol, and ethoxylated acetylenic glycol.
U.S. Pat. No. 5,665,816 discloses aqueous dispersions of polymerized units of vinyl acetate, ethylene and vinyl or acrylic ester or acrylic esters which carry epoxide groups. A cellulose ether such as hydroxyethyl cellulose is also included in partly grafted form. The resulting polymers have a glass transition temperature of from 0 to -40.degree. C. and are used as adhesives.
U.S. Pat. No. 3,562,229 discloses solvent polymerized copolymers of ethylene and vinyl esters where the vinyl esters are derived from long chain carboxylic acids, e.g., versatic acid which are mixtures of C.sub.9-11 carboxylic acids. Isotridecanoic and isononanoic are representative acids. The solvent based polymers incorporating vinyl esters of versatic acid were alleged to be superior to polymers incorporating vinyl laurate which has the same number of carbon atoms.
U.S. Pat. No. 4,322,516 discloses pressure sensitive adhesive emulsions whose copolymers comprise:
(a) 10-30 wt % ethylene, PA1 (b) 29-69 wt % acrylic acid esters of alcohols, PA1 (c) 20-55 wt % vinyl acetate, PA1 (d) 0.2-8 wt % (meth)acrylamide, and PA1 (e) 0-12 wt % other olefinically unsaturated comonomers. PA1 a high ethylene content copolymer having a Tg of from -20 to -70.degree. C. and preferably from -25 to -45.degree. C.; and, PA1 a tensile storage modulus measured at one hertz and embraced by the points on a plot of storage modulus and temperature of from 1 times 10.sup.6 and 4 times 10.sup.9 dynes/cm.sup.2 at a temperature of -20.degree. C. and less than 1 times 10.sup.7 dynes/cm.sup.2 but greater than 1 times 10.sup.3 at 50.degree. C. PA1 an ability to be coated on a variety of adhesive substrates; PA1 an ability to adhere to hard to adhere surfaces such as Mylar polyester, corona treated polyethylene and polypropylene; PA1 an ability to generate a class of water borne vinyl acetate/ethylene pressure sensitive adhesives which have an excellent balance of peel and cohesive strength while retaining dry tack; and, PA1 an ability to effect bonding vinyl and other substrates which contain plasticizers.
Tackifying agents are incorporated into the ethylene based pressure sensitive adhesives to enhance effectiveness.
U.S. Pat. No. 4,128,518 discloses a pressure-sensitive adhesive comprised of an aqueous emulsion of vinyl acetate/ethylene copolymer having an ethylene content of from 15 to 40% and a benzene-insoluble part of less than 30% by weight. The intrinsic viscosity of the benzene soluble portion is from 0.4 to 1.4 dl per gram. The polymer is prepared in the presence of a protective colloid and a polyoxyethylenic nonionic surfactant. In the polymerization process, the vinyl acetate monomer is added continuously during the emulsion polymerization process. Polymers having an ethylene content in excess of 40% were unsuited due to low tack presumably caused by high ethylene crystallinity.
U.S. Pat. No. 5,622,783 discloses vinyl acetate/ethylene/vinyl chloride adhesive wherein the polymer contains at least 40% by weight of vinyl acetate. The pressure sensitive adhesive when applied from an organic solvent is particularly adapted for adhering substrates to plasticized polyvinyl chloride.